1. Field of the Invention
The present invention relates to the preparation of solid (in particular, pulverulent and free-flowing) sodium hydrogen cyanamid (monosodium cyanamide).
2. Prior Art Statement
It is known that sodium hydrogen cyanamide can be prepared by reacting cyanamide with sodium methylate or sodium ethylate in alcohols (compare E. Drechsel; J. prakt. Chemie 11 284/53, page 311, 1875). According to German Offenlegungsschrift No. 2,358,903, it is also possible to use anhydrous sodium hydroxide as the base, instead of the alcoholates, aliphatic alcohols with 3 to 6 C atoms being used as solvents.
Although these preparation methods give highly pure sodium hydrogen cyanamide, solid cyanamide and sodium alcoholates or solid sodium hydroxide must be used as starting substances in these processes. Furthermore these prior art processes must be carried out in alcohols which are as anhydrous as possible. This makes these processes fairly troublesome and expensive and therefore of no interest on an industrial scale.
Attempts have therefore been made to react cyanamide in aqueous solution with sodium hydroxide solution, which in principle is also possible. However, the aqueous solutions of sodium hydrogen cyanamide thus obtained are not very stable and already decompose slowly at room temperature, urea, ammonia, sodium carbonate and dicyandiamide, inter alia, being formed.
The aqueous solutions must therefore be concentrated under extremely gentle conditions in vacuo in order to isolate the sodium hydrogen cyanamide formed, which moreover is very water-soluble. These gentle conditions can still just be realized on a laboratory scale with the aid of a rotary evaporator. However, considerable difficulties arise in applying this method on an industrial scale.